This invention relates to a weighing scale with reduced susceptibility to vibration.
In a typical load-cell scale, an analog voltage from the load-cell element of the scale is filtered by a low-pass analog filter and sampled at regular intervals by an analog-to-digital converter. The resultant digital signals are transmitted to circuit means for computing the weight and performing various other functions, such as calculating price.
When a weight is placed on the scale (or removed from it), successive digital signals initially increase (or decrease) continuously for a short time. In the absence of mechanical vibration and electrical noise, the value of these digital signals eventually becomes constant, and the scale is then said to be "out of motion". Computations of the weight of the object on the scale are based on this final constant value of the weight signal. The determination of when the scale is out of motion is particularly important in preventing weighing errors due to premature transmission of signals to a utilization device such as an electronic cash register (ECR) or printer.
If the scale is subject to vibration at frequencies which are passed through or only partially attenuated by the analog filter, successive digital signals may continue to fluctuate in an oscillatory manner after the initial unidirectional variation has ceased. In order to weigh the object on the scale it is then necessary to identify the constant value of the signals underlying the fluctuations. The "out-of-motion" condition is then redefined as existing when the variation in the successive signals has become small enough to permit such identification of the constant underlying value.
When signals of constant magnitude recur with sufficient frequency, a limited number of intervening signals which deviate slightly from the constant value can be ignored, and the constant or targe value of the remaining signals identified. In combination with the analog filter, this affords a limited amount of protection against moderate vibration.
U.S. Pat. No. 3,921,736 discloses a motion detector in which a weighing scale (not necessarily a load-cell scale) is designated as being in motion when a specified number of signal magnitude changes occur in one direction without an intervening change in the opposite direction. This criterion is applied without regard to any intervening signals which have the same magnitude as their predecessors. This prior art device can similarly afford a limited degree of protection against moderate vibration when used with a suitable low-pass filter or with a scale having appreciable mechanical damping.
However, with either of these approaches, a representative target value cannot be identified in the presence of substantial vibration, and therefore an accurate value of the weight cannot be computed unless the response of the scale is very sluggish.
In some commercial applications, for example, supermarket scales, rapid response of the scale when a weight is placed on it is also an important requirement. Indeed, competition of the market place impedes minimum acceptable standards of weighing speed, and there is a demand for faster weighing capabilities. Thus, the extent to which the vibration-susceptibility of a scale can be reduced by reducing the range of frequencies transmitted by the low-pass analog filter is limited by the fact that this also reduces the speed of response when a weight is placed on the scale.
For example, in one commercial supermarket scale in which the analog filter was designed to provide the best compromise between vibration susceptibility and weighing speed characteristics, a steady weight reading may be obtained in about 0.5 to 1 second (depending on the magnitude of the weight applied) when the scale rests on a rigid, vibration-free surface, such as a concrete floor. On a table or laboratory bench, this time required to reach an accurate weight is increased by up to a factor of 2 or 3 due to the vibration induced in the table by the application of the weight. In the presence of significant externally-induced vibration having a frequency below 10 Hz, it is not possible to achieve a steady weight reading at all.
The demands of the market place require the development of scales having even faster speeds of response. In such scales, the analog filter will afford even less protection against vibration.